If a fine tip is brought close to the surface of the sample such that their spacing is on the order of a nanometer, and if a voltage is applied between the tip and the sample, then a tunnel current flows. This tunnel current changes greatly, depending on the distance between the tip and the sample. The scanning tunneling microscope (STM) accurately images the surface topography of the sample, utilizing this phenomenon. Since the atomic pattern of surfaces can be observed with an STM, it has enjoyed wide acceptance. In scanning tunneling microscopy, the tip scans the surface of the sample in two dimensions to image the surface topography as described above. During the scan, the distance between the tip and the sample is so controlled that the tunnel current is kept constant. The signal used for providing this control is also employed for displaying the topography of the sample surface. A scanning tunneling microscope of this construction is disclosed in U.S. Pat. No. 4,894,538.
FIG. 4 schematically shows the prior art scanning tunneling microscope. This instrument includes a tip 2 disposed close to the surface of a sample 1. The tip 2 is mounted to an insulating plate 4 by a conductive head 3. The insulating plate 4 is attached to a z-piezoelectric unit 5 consisting of four piezoelectric elements 5a, 5b, 5c, 5d stacked on top of each other. The z-piezoelectric unit 5 acts to drive the tip 2 in the z-direction, or vertically to the surface of the sample. The unit 5 is mounted to another insulating plate 6, which is mounted to an x-piezoelectric element 7. The x-piezoelectric element and the y-piezoelectric element 8 drive the tip 2 in the x- and y-directions, respectively, and are mounted to a grounded holding member 10 by a further insulating plate 9. Electrodes 11a-11e are provided to apply drive voltages to the z-piezoelectric elements 5a-5d. Also, electrodes 11f-11h are provided to apply drive voltages to the x-piezoelectric element 7 and the y-piezoelectric element 8. Of these electrodes, the electrodes 11b, 11d, and 11g are grounded. An xy-scanning circuit 12 is installed to operate the x-piezoelectric element 7 and the y-piezoelectric element 8. The voltage for inducing a tunnel current between the tip 2 and the sample 1 is applied by a bias power supply 15. The tunnel current is amplified by a tunnel current amplifier 14. A servo circuit 13 controls the z-piezoelectric unit 5 in such a way that the tunnel current is maintained at a constant value. A sample image is displayed on a display unit -6.
In the scanning tunneling microscope constructed in this way, an electric motor (not shown) is driven to bring the tip 2 to within about 1 nanometer of the surface of the sample 1. Then, the xy-scanning circuit 12 supplies scanning signals to the x- and y-piezoelectric elements 7, 8 to cause the tip 2 to scan the surface of the sample. During this scan, the z-piezoelectric unit 5 is controlled by the servo circuit 13 in such a manner that the tunnel current fed to the servo circuit 13 via the tunnel current amplifier 14 is kept constant. The amount of elongation or contraction of the z-piezoelectric unit 5 is controlled so that the front end of the tip 2 moves up and down while following the surface contour of the sample 1. The trajectory of the front end of the tip 2 is indicated by Q in FIG. 5. The signal supplied to the z-piezoelectric unit 5 to control it is also supplied to the display unit 16. Thus, a surface image of the sample is presented on the display unit.
In this scanning tunneling microscope, when the piezoelectric unit 5 is driven, the voltage applied between the head 3 and the electrode 11a varies. The tip 2 is attached to the head 3 made of a conductive material. The head 3 and the electrode 11a are disposed on opposite sides of the insulating plate 4. Therefore, an electrostatic capacitance that cannot be neglected exists between the tip and the electrode 11a. Because the potential imposed on the electrode 11a is controlled at a high speed so that the tunnel current may be kept constant, the voltage described above also varies at a high speed. Consequently, AC coupling exists between the tip 2 and the electrode 11 a. As a result, electrical noises are introduced into the tunnel current because of the AC coupling.
The electrode 11f is disposed opposite to the electrode 11e with the insulating plate 6 therebetween. The electrode 11h is positioned opposite to the holding member 10 with the insulating plate 9 therebetween. Since the tip 2 is driven, the potential on the electrode llf is varied at a high speed by the xy-scanning circuit 12. On the other hand, the potential on the electrode 11e is also varied at a high speed by servo circuit 13. Again, AC coupling occurs between the electrodes llf and lle. The effects of the coupling are exerted on the tip via the electrode 11a connected with the electrode 11e. Similarly, AC coupling takes place between the electrode llh and the holding member 10. The effects of this coupling are exerted on the electrode 11f connected with the electrode 11h and then on the tip through the coupling between the electrodes 11f and 11e.
Furthermore, the electric field produced around the front end of the tip fluctuates because the potentials applied to the electrodes 11a, 11e, 11f, 11h, and so forth vary. Noises are introduced into the tunnel current because of the variations of the electric field. Since the tunnel current is very feeble, the quality of the sample image is severely deteriorated by the noises introduced in this way.